The present invention relates to a ball screw assembly provided with a nut having an axial end portion to which a seal member is provided.
A nut of a ball screw assembly is provided with a seal device for preventing foreign matters from invading and for preventing a lubricant filled in the nut from leaking. Various kinds of such seal devices have been proposed, including one example of a ring-shaded seal member having a spiral projection. The seal member is fitted to a ball rolling groove formed on a screw shaft and is mounted on an axial end portion of the nut, and such example has been applied widely to the commercial use. However, there is a possibility of often causing dimensional errors between the screw shaft or the ball rolling groove and the seal members when assembled, and hence, it is difficult to make a complete tight-contact between the inner peripheral surface of the ring-shaped seal member and the screw shaft.
In order to overcome such problem mentioned above, there is provided a seal device as shown in FIGS. 12A and 12B (for example, as disclosed in Japanese Utility Model Laid-open Publication No. HEI 6-6795). With reference to FIGS. 12A and 12B, a portion of a ring-shaped seal member 1 is divided into a plurality of blocks 1a, 1a, - - - , 1a in its circumferential direction. The respective blocks 1a are pushed into a screw shaft 2 so that projections 1b of the blocks 1a can closely contact a ball rolling groove 2a. Opposing divided surfaces 1c and 1d of the adjacent divided blocks 1a, 1a are inclined with respect to the radial direction of the screw shaft 2 and also inclined with respect to the axial direction of the screw shaft 2 in a twisted (spiral) direction of the ball rolling groove 2a.
According to such seal device, since the respective blocks 1a can be displaced or shifted in the radial direction of the screw shaft 2, the respective blocks 1a can surely contact the screw shaft 2 even in the presence of the dimensional error. Moreover, when the screw shaft 2 is screwed into the nut 3, foreign matters sticking on the screw shaft 2 can be scraped by the divided surface 1c of the block 1a directed to the rotational direction of the nut 3 with respect to the screw shaft 2. The scraped foreign matters can be removed outside the outer periphery of the nut 3 and outward in the axial direction in accordance with the inclination of the divided surface 1c.
Incidentally, in the seal member 1 mentioned above, the two divided surfaces 1c and 1d of the adjacent blocks 1a, 1a are substantially parallel to each other and, hence, widths of slits formed between these divided surfaces 1c and 1d of the respective blocks 1a are also substantially constant. Accordingly, the divided surface 1d has a so-called overhang shape in which the divided surface 1d gradually projects in the circumferential direction as being displaced from the inner peripheral side of the seal member 1 towards the outer peripheral side thereof, and hence, the respective blocks 1a have a shape such that a parallelogram is curved along the screw shaft 2 as viewed from the axial direction of the nut 3.
Therefore, when a pushing force is applied, as shown by an arrow A in FIG. 12A, to a portion near the divided surface 1d towards the screw shaft 2 side, it is difficult to receive such pushing force by a portion between the inner periphery of the block 1a and the screw shaft 2, and such pushing force acts to displace the divided surface 1d towards the radial central portion so that the portion near the divided surface 1d makes the width of the slit 4 narrow.
Accordingly, a force acting on the other side of the block a, i.e. the divided surface 1c as shown by an arrow B in FIG. 12A, acting in a direction apart from the screw shaft 2, arises and this force raises the inner peripheral edge 1e of the divided surface 1c from the screw shaft 2. In such a state, when the screw shaft 2 is screwed into the nut 3, the foreign matters sticking on the screw shaft 2 may be caught into the inner peripheral side of the seal member 1.
The divided surface 1c functions as mentioned above when the screw shaft 2 is screwed into the nut 3, however, the foreign matters will be pushed towards the radially central side and axially inside of the nut 3 by the opposing divided surface 1d if the foreign matters are kept in the slit 4 when the screw shaft 2 is screwed out of the nut 3. For this reason, there is a possibility of the foreign matters being caught between the seal member 1 and the screw shaft 2.